1. Field of the Invention
The present invention relates to multispectral objective lens systems. More particularly, the present invention relates to a combination of refractive optical materials that permits correction of chromatic aberrations of an objective lens system in multiple wavelength bands in the infrared.
2. Background Information
Methods are known for selecting materials for lens systems that can provide correction of chromatic aberration. U.S. Pat. No. 5,210,646 “Color Corrected Optical systems and Method of Selecting Optical Materials Therefor” discloses a method for selecting optical materials to use in designing color-corrected optical systems. Optical systems are described therein using two different types of optical materials to obtain precise axial color correction at three, four or five wavelengths with only small chromatic aberration occurring at wavelengths between the precisely color-corrected wavelengths. For example, one method disclosed in U.S. Pat. No. 5,210,646 for choosing optical materials for a two-glass three-color lens doublet involves identifying a pair of materials for which the ratio of the primary dispersion coefficient to the secondary dispersion coefficient for one material is substantially equal to the same ratio for the other material.
The article “Decade wide waveband optics” by T. H. Jamieson (SPIE Vol. 3482, 1998, pp. 306-320) discloses a method for selecting materials for a lens system having correction for chromatic aberration over a decade-wide (or larger) wavelength band in the infrared (e.g., 0.55 μm to 5.5 μm, 1.5 μm to 15 μm, or 0.55 μm to 15 μm). The method involves identifying combinations of lens materials from a given set of materials, and evaluating for each combination the average defocus at a large number of wavelengths between control wavelengths. The average defocus is used as a discriminator for selecting combinations, and the sum of the absolute optical powers of the lenses (or absolute surface curvatures) is used as another discriminator. For the 0.55 μm to 5.5 μm band, for example, the Jamieson article discloses a lens doublet of CaF and MgO and a lens triplet of clear ZnS, BaF and LiF.